Safety delay circuit



Oct. 15, 1957 ROBERTS 2,809,586

SAFETY DELAY CIRCUIT Filed Oct. '7. 1943 lllllllk fl j H w INVENTOR RICHARD B. ROBERTS BY 7/ w United States Patent 2,809,586 SAFETY DELAY CIRCUIT Richard B. Roberts, Washington, D. C., assignor to the United States of America as represented by the Secretary of the Navy Application October 7, 1943, Serial No. 505,405

1 Claim. (Cl. 102-70.2)

This invention relates to detonating circuits for explosivc projectiles and has particular reference to a novel safety-delay detonating circuit in which premature deto nation of the projectile is prevented by reason of the inclusion in the circuit of a resistor, such as a thermistor, the electrical resistance of which changes with the passage of time when current flows through the resistance and heats it. The new detonating circuit may be used for various purposes but is particularly adapted for use with electrical fuzes. Accordingly, for illustrative purposes, the invention will be described and illustrated in the form of a detonating circuit suitable for such use.

One object of the invention is to provide a novel safetydelay detonating circuit having a thermistor connected in series with a battery, a thyratron, and an electrical squib so that in case the thyratron is triggered prior to a decrease in the resistance of the thermistor to a predetermined value, insufiicient current will flow through the squib to fire the same. The thyratron may be triggered by any suitable control means. Preferably, a bleeder is connected in parallel with the thyratron and squib and is arranged so that current passes through the thermistor upon actuation of a setback switch. The thermistor becomes heated by current passing through it, and as its temperature increases, its resistance decreases until it reaches such a low value that there is suflicient current to fire the squib upon triggering of the thyratron.

Another object of the invention is to provide a novel safety-delay detonating circuit characterized by its simplicity of arrangement and which is safe and reliable in operation.

These and further objects of the invention may be better understood by reference to the accompanying drawing, in which:

Fig. 1 is a schematic view of one form of the new detonating circuit; and

Fig. 2 is a view similar to Fig. 1 showing a modification of the circuit.

Referring to the drawing, the circuit as shown comprises an electrically operated squib for detonating the usual booster (not shown) of a projectile to explode the main charge therein. One terminal of the squib is connected to a thyratron 11 having a plate 12, a grid 13, and a cathode 14, the squib being connected directly to the plate. The other terminal of the squib is connected through a thermistor 15 to the positive side of a plate battery 16 for the thyratron, the thermistor being operable to decrease its resistance as its temperature increases. A thermistor may be defined as a resistor made of material that has a very high negative temperature coeflicient, such as uranium oxide, a mixture of nickel oxide and manganese oxide, silver sulfide, boron, and others. This material may have suitable terminals embedded therein or otherwise applied thereto, and may be protected by a housing such as those used for cartridge fuses, or by a glass bulb, etc. Such devices are described in an article by Pearson in Bell Laboratory Record, of December 1940, pages 106-111, entitled Thermistors, Their Characteristics and Uses, to which reference may be made for fuller information. A conductor 17 connects the cathode of the thyratron with the negative side of battery 16. A bleeder 18 is connected between conductor 17 and the connection between the thermistor and the squib so that the bleeder is in parallel with the squib and the thyratron. A normally open setback switch 19 is disposed between the thyratron and the negative side of the battery 16, although it may be as effectively located at other places in the circuit. When a projectile utilizing this circuit is fired from a gun, the setback switch is aperated and acts to close the circuit.

The thyratron 11 is adapted to be triggered by any suitable control means, and to this end I prefer to connect the grid 13 to the output terminal of an amplifier (not shown) in a fuze. The cathode 14 may be enerized by a suitable current source 20 through a rheostat 21.

In the embodiment of the invention shown in Fig. 1, in addition to the elements described above, a delay resistor 22 is connected in series between the squib and the thermistor. A firing condenser 23 is connected in parallel with the thyratron and squib by connecting one side of the condenser between the squib 10 and resistor 22 and connecting the other side of the condenser to conductor 17.

A simpler embodiment of the invention is shown in Fig. 2 wherein the condenser 23 and the delay resistor 22 are omitted.

In the operation of the detonating circuit, the setback switch 19 closes when the projectile is being fired from the gun, with the result that current begins to flow through thermistor 15 and bleeder 18. As the current continues to flow, the thermistor becomes heated and its temperature increases, so that its electrical resistance decreases. In the circuit of Fig. 1, as long as the resistance of the thermistor is high, the firing condenser 23 cannot become fully charged, and, if the thyratron is triggered at this time, there will be insufiicient charge in the condenser to fire the squib. However, as the resistance on the thermistor decreases, the condenser becomes more fully charged until, after a predetermined length of time, it contains suflicient charge to fire the squib upon triggering of the thyratron.

In the embodiment of Fig. 2, as long as the resistance of the thermistor remains high, premature triggering of the thyratron cannot result in a suflicient flow of current to fire the squib. After the resistance of the thermistor decreases to a predetermined value, suflicient current will flow upon triggering of the thyratron to fire the squib.

I claim:

A delay circuit for an electrically-ignitable projectile, said circuit comprising a thyratron, an electrical detonator controlled thereby, a source of electrical energy for supplying electricity at adequate voltage to the anode of said thyratron and means including a resistor having a high negative temperature coeificient for limiting the output of said source initially.

References Cited in the file of this patent UNITED STATES PATENTS 1,325,048 Simon Dec. 16, 1919 1,711,693 Ruhlemann May 7, 1929 1,841,983 Ruhlemann Ian. 19, 1932 1,897,863 Ruhlemann Feb. 14, 1933 OTHER REFERENCES Pearson: Thermistors, Their Characteristics and Uses, Bell Lab, Record, Dec. 1940, pages 106-411, 

